The present invention is directed to N3-substituted imidazopyridine compounds. In particular, the present invention is directed to N3-substituted imidazopyridine compounds that are inhibitors of c-Kit proto-oncogene (also known as KIT, CD-117, stem cell factor receptor, mast cell growth factor receptor).
The c-Kit proto-oncogene is believed to be important in embryogenesis, melanogenesis, hematopoiesis, and the pathogenesis of mastocytosis, gastrointestinal tumors, and other solid tumors, as well as certain leukemias, including AML. Accordingly, it would be desirable to develop novel compounds that are inhibitors of the c-Kit receptor.
Many of the current treatment regimes for hyperproliferative disorders (cancer) utilize compounds that inhibit DNA synthesis. Such compounds' mechanism of operation is to be toxic to cells, particularly to rapidly dividing tumor cells. Thus, their broad toxicity can be a problem to the subject patient. However, other approaches to anti-cancer agents that act other than by the inhibition of DNA synthesis have been explored to try to enhance the selectivity of the anti-cancer action and thereby reduce adverse side-effects.
It is known that a cell may become cancerous by virtue of the transformation of a portion of its DNA into an oncogene (i.e. a gene which, on activation, leads to the formation of malignant tumor cells). Many oncogenes encode proteins that are aberrant protein-tyrosine kinases capable of causing cell transformation. By a different route, the overexpression of a normal proto-oncogenic tyrosine kinase can also result in proliferative disorders, sometimes resulting in a malignant phenotype. Alternatively, co-expression of a receptor tyrosine kinase and its cognate ligand within the same cell type may also lead to malignant transformation.
Receptor tyrosine kinases are large enzymes which span the cell membrane and possess i) an extracellular binding domain for growth factors such as KIT ligand (also known as stem cell factor (SCF), Steel factor (SLF) or mast cell growth factor (MGF)), ii) a transmembrane domain, and iii) an intracellular portion which functions as a kinase to phosphorylate specific tyrosine residues in proteins. Binding of KIT ligand to KIT tyrosine kinase results in receptor homodimerization, the activation of KIT tyrosine kinase activity, and the subsequent phosphorylation of a variety of protein substrates, many of which are effectors of intracellular signal transduction, These events can lead to enhanced cell proliferation or promote enhanced cell survival. With some receptor kinases, receptor heterodimerization can also occur.
It is known that such kinases are frequently aberrantly expressed in common human cancers such as breast cancer, head and neck cancers, gastrointestinal cancer such as colon, rectal or stomach cancer, leukemia, and ovarian, bronchial, lung or pancreatic cancer. KIT kinase expression has been documented in a wide variety of human malignancies such as mastocytosis/mast cell leukemia, gastrointestinal stromal tumors (GIST), small cell lung carcinoma (SCLC), sinonasal natural killer/T-cell lymphoma, testicular cancer (seminoma), thyroid carcinoma, malignant melanoma, ovarian carcinoma, adenoid cystic carcinoma, acute myelogenous leukemia (AML), breast carcinoma, pediatric T-cell acute lymphoblastic leukemia, angiosarcoma, anaplastic large cell lymphoma, endometrial carcinoma, and prostate carcinoma. The kinase activity of KIT has been implicated in the pathophysiology of several of these—and additional tumors—including breast carcinoma, SCLC, GIST, germ cell tumors, mast cell leukemia, neuroblastoma, AML, melanoma and ovarian carcinoma.
Several mechanisms of KIT activation in tumor cells have been reported, including activating mutations, autocrine and paracrine activation of the receptor kinase by its ligand, loss of protein-tyrosine phosphatase activity, and cross activation by other kinases. The transforming mechanisms initiated by the activating mutations are thought to include dimer formation and increased intrinsic activity of the kinase domain, both of which result in constitutive ligand-independent kinase activation, and possibly altered substrate specificity. More than thirty activating mutations of the Kit protein have been associated with highly malignant tumors in humans.
Accordingly, it has been recognized that inhibitors of receptor tyrosine kinases are useful as selective inhibitors of the growth of mammalian cancer cells. For example, Gleevec™ (also known as imatinib mesylate, or STI571), a 2-phenylpyrimidine tyrosine kinase inhibitor that inhibits the kinase activity of the BCR-ABL fusion gene product, was recently approved by the U.S. Food and Drug Administration for the treatment of CML. Gleevec™, in addition to inhibiting BCR-ABL kinase, also inhibits the KIT kinase and PDGF receptor kinase, although it is not effective against all mutant isoforms of the KIT kinase. Kit ligand-stimulated growth of MO7e human leukemia cells is inhibited by Gleevec™, which also induces apoptosis under these conditions. By contrast, GM-CSF stimulated growth of MO7e human leukemia cells is not affected by Gleevec™. Further, in recent clinical studies using Gleevec™ to treat patients with GIST, a disease in which KIT kinase is involved in transformation of the cells, many of the patients showed marked improvement.
These studies demonstrate how KIT kinase inhibitors can treat tumors whose growth is dependent on KIT kinase activity. Other kinase inhibitors show even greater kinase selectivity. For example, the 4-anilinoquinazoline compound Tarceva™ inhibits only EGF receptor kinase with high potency, although it can inhibit the signal transduction of other receptor kinases, probably by virtue of the fact that these receptors heterodimerize with EGF receptor.
Although anti-cancer compounds such as those described above make a significant contribution to the art, there is a continuing need for improved anti-cancer pharmaceuticals, and it would be desirable to develop new compounds with better selectivity or potency, or with reduced toxicity or side effects.
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Thus, it is desirable to develop compounds that exhibit Kit inhibition in order to treat oncology. Further, such compounds may be active in other kinases such as, for example, GIST, FLT3, Hematopoietic R-PTKs, PDGFR-beta or KDR to add efficacy in mast cell leukemias, small cell lung cancer (SCLC), mastocytosis, leukemias, myelodysplastic disorders, or angiogenic dependent diseases.